Electrowetting based liquid lenses are well known and several patents cover their general description and applications such as, for example, EP 1 816 491 A1 and EP 1 166 157 A1. The electrowetting based liquid lenses described in these patent applications, as in all current commercial applications, are based on the formulation of two phases, namely an oil and a conductive phase, the latter being water based, the conductive phase generally comprising 50 weight percent or more of water and 50 weight percent or less of organic polar components such as glycols. In addition, the two phases are generally non-miscible, and form a triple interface on an isolating substrate comprising a dielectric material, such as parylene C.
Water is generally used as the main component of the conductive phase because water provides a highly polar component in said conductive phase. Indeed, according to the Young relation, γconductive phase/dielectric=γoil/dielectric+cos θγoil/conductive phase, wherein γi/j is the surface energy between a phase I and a phase j, the wetting of the oil on the dielectric surface is favored when the conductive phase has the highest polarity. Furthermore, the use of water in the conductive phase minimizes the cross-solubility between oil and conductive phase, even at high temperature, which may result in acceptable electrowetting performances.
However, among the drawbacks of water based conductive phases are the volatility of water, particularly when the electrowetting device is used in warm or hot environment, risks of corrosion of the electrowetting device by the conductive phase, which is generally saline, and the tendency of water to degrade the electrowetting device, and thus any liquid lens or apparatus or package comprising such electrowetting device. Especially, for liquid lens applications, the use of water leads to slow evaporation of water over time and leakage of water outside the liquid lens. In addition, when too much water is lost, an air bubble will appear in the liquid lens, which renders said lens ineffective. For example, the liquid lenses described in EP 1 816 491 A1 present failures due to air bubble apparition in the water-based conductive phase after long-term storage at high temperature. Another drawback of water-based conductive phases, in industrial applications, is the requirement of having a water-based conductive phase having a freezing point below −20° C., which is difficult to be achieved without impairing other properties of the water-based conductive phase.
There are in the patent literature other general descriptions of conductive phases, which may be used in electrowetting devices (see, for example WO 2004/099845 A1). However, to the best of our knowledge, all patent literature discloses water-based conductive phases.
Nevertheless, the academic literature provides some exemplary non-aqueous formulations for electrowetting devices. For example, Heikenfeld et al. describe non-aqueous conductive phases for electrowetting applications (see J. disp. Technol. 2011, 7, 649-656; Langmuir 2009, 25, 12387-12392). More particularly, these non-aqueous conductive phases are composed of DMSO, ethylene glycol, formamide, γ-butyrolactone, N-methylformamide, propylene carbonate, propylene glycol, or 2-pyrrolidone and further comprise a small quantity of large size ions, such as the anionic surfactant sodium dodecyl sulfate (SDS) dissolved therein. In order to avoid the problem of charge injection in the electrowetting process, large size ions are used in low concentration (less than 1 weight % and preferably less than 0.5 weight %) in the non-aqueous conductive phase. Also, the oils described in the papers are dodecane and a polydimethylsiloxane, which are highly insoluble in the polar media. However, these oils and the non-aqueous conductive phases are not density matched and their optical indexes are not suitable for liquid lens applications having the required quality.
Lastly, a paper from X. Hu, S. Zhang, Y. Liu, C. Qu and L. Lu describes non-aqueous conductive phases based on ionic liquids (see Applied Physics Letters 211, 99, 213505). However, non-aqueous conductive phases based on ionic liquids are inadequate because they do not satisfy the requirement of having a freezing point below −20° C. Indeed, ionic liquids are solids at room temperature.
Accordingly, there exists a continuing need to provide highly reliable electrowetting devices.